The Mission Bay spot is one of two locations that some arena supporters point to as an option if plans have to change.

Photo: Jason Henry, Special To The Chronicle

The Mission Bay spot is one of two locations that some arena...

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Lot A near AT&T park is also considered a possible backup location for the Warriors arena.

Photo: Lea Suzuki, The Chronicle

Lot A near AT&T park is also considered a possible backup...

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A rendering shows an artist's conception of the proposed Mission Rock development, which will put about 2,000 residents and 7,000 office workers in a new mixed-use development the Giants hope to build on the parking lot for AT&T stadium.

Photo: San Francisco Giants

A rendering shows an artist's conception of the proposed Mission...

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Third and 16th Streets, a proposed site for the new Warriors Arena, in San Francisco, Calif., Monday, March 3, 2014

Photo: Jason Henry, Special To The Chronicle

Third and 16th Streets, a proposed site for the new Warriors Arena,...

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Traffic cones are lined up in parking Lot A, just south of AT&T Park, in San Francisco, Calif. on Thursday, Dec. 26, 2013. The site is one of several that has been considered as a possible location for a new sports arena.

The San Francisco Giants announced their development plans for seawall lot 337. Viewed is an artist rendering of China Basin Park.

The San Francisco Giants announced their development plans for...

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Aerial view of where the Golden State Warriors proposed arena would have been located at Pier 30-32.

Photo: Warriors/Snohetta/steelblue

Aerial view of where the Golden State Warriors proposed arena would...

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A public plaza backed by a restaurant will replace what had been a high staircase and an entrance to a parking garage on the northwest corner of Piers 30-32. A gently sloping walkway flanked by shops and restaurants would lead up to the arena. The parking garage entrance was moved mid block. The plan also calls for a fire station and water taxi dock on that corner of the site.

Photo: Warriors/Snohetta/steelblue

A public plaza backed by a restaurant will replace what had been a...

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Hard-surfaced terraces facing south were replaced with sloping grass. The area would be part of 7.6 acres of public open space on the site.

Photo: Warriors/Snohetta/steelblue

Hard-surfaced terraces facing south were replaced with sloping...

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The Warriors were planning on building their new stadium at piers 30/32 in San Francisco.

Photo: Sonja Och, The Chronicle

The Warriors were planning on building their new stadium at piers...

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Seagulls congregate on Piers 30-32 in San Francisco. The site was preferred by the Golden State Warriors as a possible location for a new sports arena.

Photo: Paul Chinn, The Chronicle

Seagulls congregate on Piers 30-32 in San Francisco. The site was...

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The development that the owners of the Golden State Warriors which had wanted to build on Piers 30-32. The large circle is the proposed 18,000 seat area. The block on the inland side of the Embarcadero would include a hotel and a residential mid-rise.

Photo: Courtesy Golden State Warriors

The development that the owners of the Golden State Warriors...

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Red's Java House would have been moved south on the Embarcadero to make room for the new Golden State Warriors arena at Piers 30-32. Image courtesy the Warriors.

Red's Java House would have been moved south on the Embarcadero to...

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A rendition of the proposed Golden State Warriors arena at Piers 30-32. Courtesy the Golden State Warriors

A rendition of the proposed Golden State Warriors arena at Piers...

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In this file photo, Lawrence Stokus stops to talk to a security guard at Piers 30-32 about the proposed Warriors arena in San Francisco. Stokus lives in the neighborhood and would rather see the land be used as a park.

(03-05) 11:27 PST San Francisco -- The Golden State Warriors say their waterfront arena plan is "full speed ahead," but supporters are subtly eyeing alternate sites in San Francisco as financial and political uncertainty chew at the team's current proposal.

The Warriors say they are moving forward with their original proposal for an 18,000-seat arena at Piers 30-32, just south of the Bay Bridge, while officials within Mayor Ed Lee's administration and others have been quietly discussing backup sites as the cost to rebuild the piers has doubled to $180 million.

Among the options are the main parking lot across from AT&T Park, city-owned land currently leased to the San Francisco Giants, or some of the 14 acres in Mission Bay that had once been planned as the corporate campus for Salesforce.com.

Both sites had been considered earlier and shelved because of various drawbacks, but they've re-emerged now as the Warriors and the Giants, both would-be developers, grapple with a ballot initiative headed to voters in June designed to limit high-rises along the waterfront. If passed, it could kill or alter each team's separate plans, and without a viable backup, Lee risks seeing an arena that he once called "my legacy project" evaporate before his eyes.

The Chronicle reported Monday that the Giants and the Warriors, while not in direct talks, were considering discussions about putting the facility on what is now the main parking lot across from AT&T Park, a site former Mayor Gavin Newsom once called a "natural" fit for an arena.

Giants' plan for site

The lot is currently getting an in-depth review as an alternate site to Piers 30-32 in the arena's environmental impact study. The Giants already have an exclusive negotiating agreement with the city and extensive plans to develop the 13.6-acre parking lot, also known as Seawall Lot 337, into an urban village with towers up to 380 feet. But there seems to be little appetite among voters for such heights.

The two sports franchises, though, have had prickly relations since the Warriors first announced in May 2012 that they planned to decamp from Oakland to San Francisco's waterfront.

Another option getting initial discussion, sources say, is part of the 14 acres in Mission Bay that Salesforce bought in November 2010 for about $250 million to hold its corporate campus. The San Francisco software giant announced in February 2012 that it was shelving its plan to build there. A portion of the site, bounded by Terry Francois Boulevard and 16th, Third and South streets, across from a bayfront park, could hold an arena.

A Muni light-rail line stops next to the site on Third Street, and two large parking garages nearby that are mostly used during the day would be available for evening events.

Despite its proximity to the bay, the site is not on Port of San Francisco property, and therefore it is not subject to the ballot measure that limited-growth advocates qualified for the June ballot. The measure is being challenged in court, but if it goes to the voters, even its opponents predict that it will pass. It would require voter approval for any development on port property to exceed current height limits, which range from essentially zero on the Giants parking lot, which is zoned for recreational use, to 105 feet.

The land cost $248 million, according to a Salesforce filing with the Securities and Exchange Commission in November. But that figure doesn't include $23.3 million for "perpetual parking rights" in at least one nearby garage, or interest costs and property taxes since the purchase.

"The total carrying value of the land, building improvements and perpetual parking rights was $321.1 million" as of Oct. 31, according to the filing.

That's a far cry from the $180 million the Warriors are looking at to rebuild the 13-acre expanse of concrete and aging pilings at Piers 30-32, which the city would continue to own and lease to the team for 66 years.

Flexible price?

Officials, though, have suggested that there may be wiggle room in Salesforce's asking price. The mayor has ties to Salesforce CEO Marc Benioff, a generous philanthropist whom Lee regularly cites as the model tech executive. Also, the costs of a land deal could be reduced if Salesforce is given naming rights for the arena, an observer said. Recent naming deals, such as the one for New York's Barclays Center, were worth about $200 million over 20 years. The Warriors, though, are already planning on applying naming rights revenue to the project's bottom line.

Similarly, P.J. Johnston, a Warriors spokesman, said: "We're just not going to discuss other sites. Our focus remains on Piers 30-32, which we believe is a spectacular, transit-rich and logical place for the event pavilion and eight acres of new public parks."

$20 million spent

The team said it has already spent $20 million on developing the piers, where it would also stand to recoup up to $120 million from the city in revenue linked to the development.

Other income from the site, though, is based on building a 175-foot condominium tower across the street, where the height limit is 105 feet. Analysts say voters, anxious about evictions and the city's cost of housing, are in no mood to approve luxury condos.

Lee is expected to meet within a week with top Warriors executives for an update on the team's plans.

"I still personally believe that the site that they've chosen, Piers 30-32, is the best for transportation," Lee said. "I think they're as enthusiastic as when they announced it. They knew it was going to be hard."